Effect of active and passive cooling on the thermo-hydrodynamic behaviors of the closed-loop pulsating heat pipes

This paper reports the results of an experimental study when a closed-loop pulsating heat pipe (CLPHP) was subjected to active and passive cooling for its thermal performance and flow regimes. The active cooling was done using forced convection while the passive cooling was done by free convection. The results take into account the effect of convection sources on the start-up pulsations, dry-out conditions, critical heat flux, thermal performances, flow regimes, and bubble behaviors of CLPHP using a uniform heating pattern. The experiments are performed under normal gravity conditions, with methanol employed as a working fluid. A high-speed camera is used to record the internal flows for various values of heat input (Q(in)). Experimental results indicate that the free convective CLPHP exhibits delayed start-up pulsation and early evaporator dry-out, while the forced convective CLPHP has an early start-up pulsation and delayed dry-out condition. Slug-plug flow and oscillatory annular flow are dominant for the free convective CLPHP, while slug-plug flow, elongated plug flow along with higher amplitude, and oscillatory annular flow are dominant for the forced convective CLPHP. Liquid thin films are dominant for both free and forced convection at high Q(in). The bubbles are arranged in irregular patterns for free convection, while active oscillations with regular bubble patterns are observed for the forced convective CLPHP. The condenser temperature affects bubble displacement, thereby leading to higher velocity for the free convective CLPHP. The time-averaged acceleration of the bubbles increases with an increase in Q(in). The present analysis will be helpful to design and develop a CLPHP based on its thermo-hydrodynamics properties for engineering applications. (C) 2020 Elsevier Ltd. All rights reserved.